Port: A port is a specific place for being physically connected to some other device, in this case it is the jack used to connect your analog wired or wireless telephone to the device to be translated into a digital signal.

FXS: (Foreign eXchange Station) The interface provided by the telephone company to its customers, which includes dial tone, power and ring voltage. The telephone jack on the wall is an FXS interface.

PSTN: The public switched telephone network (PSTN) is the worldwide collection of interconnected public telephone networks that was designed primarily for voice traffic, it is both commercial and government-owned.
It's also referred to as the Plain Old Telephone Service (POTS)